I
A. V. GROSSE and J. B. CONWAY‘ Research Institute, Temple University, Philadelphia, Pa.
Combustion of Metals in Oxygen The basic aim of the work was to develop a technique for producing and utilizing high temperature sources. Combustion of aluminum yields a source of high intensity thermal radiation. Powdered metal-oxygen flames are highly effective in cutting through thick sections of concrete and ceramic materials.
values differ for combustion in fluorine or some other atmosphere). The heats of combustion of some common combustibles and typical metals listed in Table I are from the recent compilation of Brewer ( 3 ) . O n a pound basis the metals liberate about as much heat as carbon, methane, and acetylene; hydrogen is very high because of its low atomic weight. O n a gram-mole or gram-atom basis the same is true, hydrogen now being near the bottom of the list.
The first experiments on combustion of metals were those of von IngennHausz in 1782, in which heated spirals of iron and steel were plunged into oxygen (26). These experjments demonstrated that certain metals could be made to burn in oxygen to produce highly luminous flames generating large quantities of heat and light. These characteristics of metal combustion have been primarily responsible for numerous practical applications of this fundamental reaction. T h e operation of a photo-
IN
’
1948 the Research Institute of Temple University established a High Temperature Laboratory to study and develop methods for producing high temperatures (about 3000’ (2.). The basic objective was to establish techniques for producing, confining, maintaining, and controlling high temperatures, and ultimately to apply this experience in studying chemical and physical phenomena in this temperature region. Most previous work has dealt with chemical reactions of substances in the gaseous, liquid, or solid phases (2, 7-77, 77, 32, 33). The combustion of metals was convenient and effective for producing high temperatures and was studied in considerable detail. The dictionary defines combustion as “any chemical process accompanied by the evolution of light and heat.” Oxygen need not be one of the reacting substances; the reaction of hydrogen with fluorine is as truly a combustion as the reaction of hydrogen with oxygen. The combustion of metals therefore designates reactions of metals which evolve heat and light. The work described is limited to combustion in which the metals react with molecular oxygen. Reactions with other gases, including ozone, are described in separate articles. Although metals are not generally looked upon as fuels, they must be classified with such solid combustibles as coal and wood. Their combustion differs from that of wood in the formation of reaction products (metallic oxides) which are solid a t room temperature. Their heats of combustion and ignition temperatures, however, are of the same order of magnitude as those of various solid nonmetallic fuels (interpreted as applying to reactions with oxygen; these Present address, Aircraft Nuclear Propulsion Department, General Electric Co., Evendaie, Ohio.
Table 1.
Fuel
Combustion Product
Hz
HzO(1)
C CH4 CzHz
COa, HzO COz, Hz0
Li Na Be Mg Ca Sr Ba B AI Th Ti Zr Fe co Ni Si Sb Bi Hf
Liz0 NazO Be0 MgO CaO Sr0 BaO BaOs AlzOs ThOz Ti208 ZrOz FeO coo NiO Si02 SbzO8 Biz03 Hf 02
co
coz coz
Heats of Combustion
At. or Mol. Wt.
Kcal./G. Mole Fuel
2.016 12.0 16.03 26.02 28.0 6.94 22.997 9.02 24.32 40.08 87.6 137.4 10.8 26.97 232.1 47.9 91.2 55.85 58.94 58.69 28.06 121.76 209.0 178.6
68.3 94.05 212.7 310.5 67.6 71.3 49.7 147.0 143.7 151.7 140.8 133.0 152.7 200.1 293.2 181.5 261.8 63.2 57.0 58.0 210.2 83.5 69.0 266.0
I
Kcal./Gram B.T.U./Lb. Fuel Fuel 33.9 7.83 13.3 11.9 2.41 10.25 2.16 16.3 5.9 3.79 1.61 0.97 14.1 7.43 1.26 3.79 2.87 1.13 0.97 0.99 7.5 0.69 0.33 1.49
61,000 14,100 24,000 21,400 4,340 18,500 3,900 29,400 11,600 6,820 2,900 1,750 25,400 13,400 2,300 6,820 5,160 2,000 1,750 1,780 13,500 1,240 594 2,680
Kcal./
Gram Op 4.27 2.94 3.32 3.88 4.22 8.93 6.25 9.2 9.0 9.4 8.8 8.4 6.4 8.3 9.15 7.6 8.2 3.9 3.6 3.6 6.6 3.5 2.88 8.3
History of Combustion of Metals
Author
Lit.
Baker and Strong Cueilleron and Scartazzini Scartazzini Bunsen and Roscoe Forsythe and Easley Van Liempt and de Vriend Cassel, Das Gupta, and Guruswamy Coffin
(1) (13)
Hart and Tomlinson Hartmann, Nagy, Brown, and Jacobson Rasor
(18)
Wolfhard and Parker
(34)
Zenghelis
($6)
Remarks Burner fed with finely divided A1 powder dispersed in 02
($0)
(4)
(16)
Measurements of radiant energy emitted by combustion of metals
(366)
(6) (6)
(19-91) (39)
Factors affecting flame propagation through dust clouds Burning times of Mg ribbons in various atmospheres Use of finely divided metals in explosives Ignition characteristics and limits of flammability of metal dust dispersions Patent for use of powdered metal as fuel for propulsion of submarines Optical measurements on stationary flame of A1 and Mg flakes suspended in air, color temp. of 3600’ C. recorded Burning AI in MgO crucible, qualitative measurements of combustion temperature
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graphic flashbulb, for instance, is based on the release of large quantities of radiation (visible) during such a reaction. Fundamentals of Combustion
Figure 1. Aluminum combustion i s unique. The brilliant molten pool of burning metal on a background of molten aluminum oxide has been called “the skating sun”
M.P.,
Na Mg
Ca A1
Ti Zr
O
C.
(93) 97.8 650 850 660 1730 1845
Table II. Physical Properties B.P., O C. M.P., O C . (33)
883 1103 1690 2500 3260 4900
(3)
NasO MgO CaO
A1203
Ti203 ZrO2
B.P., O C. (3)
...
d1770
2800 2590 2050 2100 2690
3100 3500 3500 3000 4300
The term “fundamentals of combustion” can properly be interpreted as applying to the mechanism of ignition, the mechanism of the reaction with oxygen, the inorganic chemistry of the products formed in the combustion, and the temperatures obtained during combustion. Ignition temperatures and flame temperatures are discussed here in some detail, but not the mechanism of combustion or inorganic chemistry. The mechanism (kinetics) of metal combustion reactions has been studied only very slightly; Brewer (3) has reviewed the inorganic chemistry of the important metallic oxide species at high temperatures, but their role in the combustion process has been studied in only a few instances. Combustion must be distinguished from the high temperature slow oxidation of metals which takes place below ignition temperature (24). The most striking difference lies in the rates of heat generation, although this can represent no rational basis for comparison, as by definition they take place a t different temperatures. Another important difference lies in the rate laws governing the reactions. As the temperature increases the slow oxidation proceeds logarithmically, parabolically, and then linearly (24))although this has not been confirmed for all metals. The rate of reaction during combustion is constant with time for a given set of conditions, as long as the surface area remains constant. “Skating Sun” Phenomenon
Figure 2. Aluminum oxide smoke from this spherical pot furnace indicates high oxygen rates in use
664
INDUSTRIAL AND ENGINEERING CHEMISTRY
The general appearance of the combustion process is determined by the physical and chemical properties of the metal and its oxide. Metals such as sodium, magnesium, and calcium have low boiling points (Table 11), and considerable volatilization takes place, causing combustion at considerable distances above the surface of the metal. Aluminum, titanium, and zirconium have high boiling points and, under ordinary conditions. the combustion zone is close to the surface. The oxides of magnesium and caIcium are high melting (close to combustion temperatures) and very little melting is noted within the furnace. The melting point of aluminum oxide is much below the combustion temperature of aluminum; hence, a pool of molten oxide accumulates within the furnace as combustion proceeds. The same is true of titanium and zirconium, although these metals appear to have higher densities than their oxides and to sink
M E T A L C~OMBUSTIONIN OXYGEN into the molten oxide when it accumulates. The density of aluminum is lower than its oxide; hence, it floats on the surface of the molten oxide during combustion. The combustion of aluminum is unique, not so much because it melts before it ignites but rather because of the extreme brilliance of the boiling pool of burning metal as it floats within the furnace on the molten aluminum oxide (combustion product). T h e combustion can be propagated indefinitely by feeding aluminum (in rod form) to the pool of burning metal, although if the metal rod is added too rapidly the pool is chilled below its ignition temperature. This combustion behavior has been termed the “skating sun” phenomenon, as the brilliance of the burning pool of metal can be compared to that of the sun in the heavens. A typical “sun” is shown in Figure 1; the dark background is molten aluminum oxide. During combustion the atmosphere inside the furnace is very clear, very little smoke being produced as long as the oxygen rate is low. The sun phenomenon is therefore clearly visible during the entire combustion process. About 90 grams per minute of aluminum is consumed for a “sun” 8 inches in diameter. As the oxygen rate is increased, the combustion rate increases and the pool of liquid aluminum begins to boil vigorously, causing large quantities of aluminum oxide smoke to be generated. The sun is no longer visible and the conditions within the furnace are highly turbulent. The furnace in Figure 2 is being run at high oxygen rates, as indicated by dense clouds of smoke. The brilliance of these suns can be seen in Figure 3, which shows a water-cooled furnace with a sight-glass on top. The furnace shown in Figure 4 was constructed entirely of aluminum oxide bricks and had a n internal volume of about 1 cubic foot. With these furnaces suns 18 inches in diameter were produced. Theoretically there is no limit to the diameter of this burning pool of aluminum. The temperature of the aluminum in the sun has been estimated as close to the boiling point of the metal (2500’ C.). (The temperature of the AI-02 flame, above the surface of the boiling metal, is about 3500’ C.) However, the sun will die out rapidly if air is suddenly fed to the furnace instead of oxygen. This seems to indicate that large masses of aluminum cannot be burned in air. Aluminum has been the only metal to exhibit the sun phenomenon in such clearly defined form (at pressures ranging from 4 mm. to 150 p s i . ) . I t would seem that any metal whose ignition temperature is above its melting point would exhibit this behavior. However,
Figure 3. The sight glass on top shows the brilliance of suns produced in a watercooled furnace
comlpstion characteristics are influenced to a great extent by the properties of the metal oxide. For instance, for sun formation the molten metal must be less dense than the molten oxide, and its boiling point high. It is expected that beryllium will show the same phenom. enon. The combustion of aluminum has also been carried out in centrifugal reactors (F of a cylinder of plain with 2 inches of aluminum oxide. Two end plates, also lined with 2 inches of refractory, completed the enclosure. Two small holes were provided a t each end of the furnace, one for feeding fuel and oxygen and the other for observation and to allow combustion products to
.
escape. The combustion was started as described and aluminum rods were fed into the furnace. When a large sun was obtained, the furnace was rotated a t 300 r.p.m., throwing molten aluminum (in the form of skating suns) onto the walls by centrifugal force. Aluminum rods were fed until the entire inner wall was covered with one continuous sun. The heat produced was thus confined within the rotating chamber and was sufficient to produce aluminum vapor when the oxygen rate was low. At high oxygen rates (300 liters per minute) combustion was so rapid that the boiling mass of aluminum seemed to fill the entire chamber. A situation similar to that in the atmospheric pot furnace at high oxygen rates was thus produced;
Figure 4. Suns 18 inches in diameter were made in aluminum oxide brick furnaces like this VOL. 50, NO. 4
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,
(2 atomic weights) or 7400 calories per gram. In heterogeneous equilibria when two solid phases (each of unit activity) and one gas phase are involved, as in Equation l: the equilibrium constant, K , has the form :
where Po2 is the partial pressure of oxygen in equilibrium with the metal and oxide phases at a given temperature, For the reverse reaction the expression is : K = (P0.t)~
Figure 5. Centrifugal reactors (front end shown here) produce aluminum vapor a t low oxygen rates
a brilliant flame formed at the exit port of the furnace (Figure 6): so intense as to make it impossible to identify the center portion of the end plate shown in Figure 5. The heat from the flame melted the refractory material around the exit port in a very short time. When the oxygen flow was stopped, the heat contained in the furnace was still sufficient to cause the aluminum to remain close to its boiling point. These metallic vapors burned in air with a yellow flame as soon as they left the exit port of the furnace. Thermodynamics of Combustion The general equation representing the stoichiometry of all metal-oxygen reactions is: xM(s)
+ y Odg)
-+
MDds)
Figure 6.
666
(1)
where x atoms of solid metal M reacr. with y moles of gaseous oxygen to produce 1 mole of solid metallic oxide, M,Oz,. For aluminum the reaction would be:
The heat of the reaction at 298' K. in cases of this type is equal to the heat of formation of the solid oxide at 298' K., and at 298' K., the free energy change of the reaction is equal to the free energy of formation of the solid oxide. Heats of formation at 298' K. for the more common metallic oxides are given in Table 111, with other pertinent properties. These heats of formation are used to calculate heats of combustion-the heat of combustion of aluminum is 400,200 calories divided by 54 grams
Centrifugal reactor in operation
INDUSTRIAL AND ENGINEERING CHEMISTRY
(4)
In Equation (4) the equilibrium ox)-gen pressure is referred to as the dissociation pressure of the oxide and is indicative of its thermal stability. Oxides with low dissociation pressures-e.g., ZrO2, ThOa-can be heated to very high temperatures before they decompose; those with high dissociation pressures decompose at relatively low temperatures. The oxides of silver, gold, and platinum have high dissociation pressures at low temperatures (silver oxide, 0.5 atm. at 400" K . ) ; hence it is easy to understand why they are so resistant to oxidation. Some metallic oxides have very low dissociation pressures even at very high temperatures. The corresponding metals therefore may oxidize rapidly at elevated temperatures to produce very stable oxides. This property coupled with a high heat of combustion suggests the possibility of producing very high temperatures by combustion of these metals in oxygen. If these products are completely stable, all the heat release can be used to heat these materials to very high temperatures, as the combustion products Mill not dissociate. The basic requirements, then, for a metal-oxygen reaction to produce high temperatures are : high heat of reaction; high oxide stability-low dissociation pressure even at high temperatures; high reaction rate; and low heat capacity of reaction products. The need for high reaction rates is obvious when it is considered that the slow atmospheric oxidation of iron and aluminum liberates the same quantity of heat per unit mass of metal as the combustion, yet in the former the heat is dissipated as fast as it is produced and the temperature remains low. Low heat capacity is usually of little importance, for the heat release is more than enough to heat the reaction products to their dissociation temperatures and therefore the temperature attained is determined more by this factor than by the heat capacity of the products. Heat capacity could be important, but ordinarily its effect can be disregarded. The calculation of the temperature obtainable in a given combustion process is simple, once thermodynamic data are
M E T A L COMBUSTION IN OXYGEN available. The procedure can be illustrated by considering the combustion of carbon. This reaction
c (5)
+
0 2
( 9 ) -t
coz ( 9 )
Table 111.
Oxide
Weight
Liz0 Na2O K20 Rb2O Be0 MgO CaO SrO BaO BzOs AltOi Tho2 Ti203 ZrO2
29.9 62.0 94!2 186.9 25.02 40.3 56.1 103.6 153.4 69.6 101.9 264.1 143.8 123.2 210.6 158.0 143.0 231.9 70.9 231.5 74.9 74.7 122.7 227.2 143.1 231.8 81.4 128.4 216.6 60.1 150.7 223.2 583.04 466.0
-142.6 - 99.4 - 86.4 - 78.9 - 147.0 -143.7 -151.7 -140.8 -133.0 -305.4 -400.2 -293.2 -363.0 -261.8 -266.0 -270.0 - 180.3 -200.1 - 92.0 -268.0 57.0 - 58.0 - 21.0 - 32.0 - 39.8 - 7.3 - 83.2 - 62.2 - 21.7 -210.2 -138.8 - 52.4 -334.2 - 46.0
(5)
is similar to a metal-combustion reaction, in that the heat of the reaction is equal to the heat of formation of the reaction product. The heat of formation of carbon dioxide (g) at 298" K. is -94.052 cal. per gram mole. As this heat will be available to raise the temperature of the gaseous product, the temperature obtained by the combustion is given by the upper limit on the following integral:
Mol.
HfOz Cr2Oa
where C, is the heat capacity of carbon dioxide (g) in calories per mole-degree. A temperature close to 8000" C. is obtained by solving this equation. This is undoubtedly in error, as the maximum temperature actually obtained is about 2500" C. The discrepancy lies in the fact that Equation 6 does not allow for dissociation of the carbon dioxide at elevated temperatures. When carbon dioxide is heated above 1800" C. its dissociation becomes appreciable (16% at 2200' C. and 56% at 2800" C. and 1-atm. pressure) and the heat available for raising the temperature is correspondingly reduced. This illustrates the importance of reaction-product dissociation in making combustion temperature c?lculations. The temperatures attainable in the combustion of metals can be calculated using integral equations similar to ( 6 ) , if additional terms representing the latent heats of fusion and vaporization of the metallic oxide are added (accurately the heat effects due to any allotropic transformations must also be added). The data cited in Table I11 would be applicable, with heat capacity data compiled by Kelley (22). These calculations can also be made graphically using a n enthalpy or "heat content" chart to plot the heat content of the reaction products, in calories per mole above 298" K . (usually designated HT-H298),us. temperature. Data of this type have been tabulated by Kelley (22) for numerous substances. The heat of the reaction is then located on the ordinate and a horizontal line is drawn from this ordinate value until it intersects the curve. The abscissa of this point of intersection gives the temperature desired. This type of solution is illustrated by the plot for aluminum oxide shown in Figure 7 (heat of vaporization was estimated to be 100 kcal. per mole and heat capacity of the liquid 35 cal. per moledegree). As the heat of the reaction is 400,200 cal., it would seem that the combustion temperature should be much
Properties of Metallic Oxides Heat of Formation Kcal./G. Mole (5)
MOO8
W Oa MnO FeaOI coo NiO PdO PtOn CUZO AgzO ZnO CdO HgO Si09 SnOn PbO Sb406 Biz03
-
higher than 4000' K. Once again, however, dissociation becomes important and allowances for these effects must be made, as in the dissociation of the more common gaseous molecules such as carbon dioxide and carbon monoxide (37), although the dissociation data are usually more uncertain for the gaseous oxides. Some attention must also be given to the vaporization process itself. Brewer (3) reports that most metallic oxides vaporize exclusively by decomposition to the gaseous elements, although some form gaseous oxide molecules. Thus, the heat of dissociation must be considered, as it absorbs some of the heat of the reaction and dehreases the amount available for raising the temperature. Only about 140,000 cal. are required to heat aluminum oxide to its boiling point (Figure 7 ) . This emphasizes the relative importance of the heat capacity of the reaction products. The heat amounting to the difference between 400,200 and 140,000 cal. must be interpreted as being used to vaporize and decompose the aluminum oxide at its boiling point (aluminum oxide decomposes on vaporization). The combustion temperature is thus limited to the boiling point of the oxide. This is the case in many metal-oxygen systems, but
Melting Point, a
K. (5) 2000 1193 800 900 2820 3075 2860 2730 2196 723 2323 3225 2400 2960 3063 2710 1068 1746 2058 1870 2078 2230 d1150 d 750 1503 d 460 2248 1500
>
... 1973
> 2200
1163 928 1090
Heat of Fusion, Kcal./ Mole (16) 18.0 11.2 7.2 8.5 17.0 18.5 12.0
...
13.8 5.5 26.0
...
38.4 20.8
... ... 12.5
15.0 13.0 33.0 9.6 12.1
Boiling Point,
O K. (5) 2600 1550 (sublimes) d1750 dl600 d4300 3350 3800 3500 3000 2520 d3800 4670 3300 4570
d33OO 1530 2100 d3400 d2060
...
... ... ... ... ... ... 13.4 ... ... ... 12.5 d22.50 ... d1750 ... d 750 2.1 d2500 ... ... 2.8 1745 30.0 1698 6.8 ...
each reaction must be evaluated independently. The temperatures so calculated are called adiabatic combustion temperatures, as no account is taken of heat losses from the reaction zone. Where heat losses occur, actual temperatures will be less than those calculated. Metals whose combustion will produce high temperatures can be predicted systematically from considerations based
TEMPERATURE
O K
Figure 7. Enthalpy of aluminum oxide, HT-H29skcal./mole VOL. 50, NO. 4
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667
Figure
8.
Periodic system and melting,
M, decomposition, D, and boiling points, 5,
of metallic oxides tower line gives heats of formation a t 25" C. in kcal. per mole formula weight. highest flame temperature in each period are cross-hatched
ATOMIC
Figure
9.
a
Heats of formation (at
ATOMIC
NUMBER
298" K.)
NUMBER
OF M E T A L
of oxides of metals vs. atomic number
OF
ELEMENT
Figure 10. Adiabatic combustion temperature, of element
668
Metals producing
INDUSTRIAL AND ENGINEERING CHEMISTRY
' K.
at 1 .O atm., vs. atomic number
on the Periodic Table. In Figure 8, wherever possible, the importanr oxide has been placed in the position ordinarily occupied by the metal. The number below the oxide represents the heat of formation of the oxide at 298' K. in kilocalories per gram-mole. The number above the oxide represents the normal boiling temperature in O K. of the oxide when preceded by B, and decomposition temperature at a pressure of 1 atm. when preceded by D. A number preceded by M indicates the melting point of the oxide and is used when no other data are available. The data used in Figure 8 are due to Brewer (3). In Figure 8 the oxides with high heats of formation are located on the left side of the table. A plot of the heat of formation of the important oxide as a function of the atomic number of the metal indicates an interesting periodicity. This familiar type of plot (Figure 9) shows a low point at all the alkali metals (and the noble metals) and a maximum at the metals of the boron-aluminum group. This classification defines a group of metals in the boron-aluminum group whose oxides have very high heats of formation, very low dissociation pressures, and hence are extremely stable. These metals would be expected to burn in oxygen to produce very high temperatures. The beryllium-magnesium group also exhibits these same characteristics and can be counted upon to produce similar temperatures. Beryllia, for instance, decomposes when heated to 4300" K. The oxides of the alkali metals have fairly high heats of formation but, except for lithium, their oxides are not very stable. Lithium represents the only promising metal in this group from the high temperature point of view, but it is limited to about 2600" K. Other metals worthy of consideration are titanium, zirconium, hafnium, thorium, and possibly protactinium. These considerations locate a diagonal band of elements (cross-hatched in Figure 8), which embraces all the important metals whose combustion in oxygen produces high temperatures. Although it is not possible to predict with any accuracy which metal will produce the highest temperature during combustion, lanthanum, zirconium, hafnium, or thorium at a pressure of 1 atm. should burn in oxygen to produce a temperature close to 4800' K. The theoretical adiabatic combustion temperatures for some metal-oxygen reactions are listed in Table IV and plotted in Figure 10. These temperatures were calculated according to the methods discussed. In some cases thermodynamic data were not available and the temperatures were estimated. In the absence of heat capacity data for the liquid oxides, those of the solid may be used, if it is remembered that ,heat
METAL COMBUSTION IN OXYGEN capacities for the liquid phase are usually a little higher than those for the solid phase. The biggest error is made when data are not available on boiling temperature, latent heats of vaporization, or the oxide species which exist at high temperatures. As Figure 10 shows, the combustion temperatures are periodic functions of the atomic number of the element. The maxima are the metals of Groups 11, 111, and IV. In the first period beryllium has the highest combustion temperature; in the second, aluminum; in the third, calcium (and probably scandium); in the fourth and fifth, zirconium and hafnium, respectively; and in the last, thorium. The minimum combustion temperatures are obviously obtained with the nonmetals, the absolute or trivial minima being the noble gases, as they do not combine with oxygen. There are secondary minima in the transition group metals, occurring with the monetary metals-copper, silver, and gold-and the last members of the noble metals such as nickel, palladium, and platinum; intermediate maxima, probably at germanium, tin, and bismuth, also occur with the transition metals. Figure 10 gives a general correlation of the combustion temperatures of metals. Some of the values are only estimates (usually indicated with a ?), but the whole relationship will not change appreciably when better data are available. Flame temperatures are a function of total pressure. High pressures increase the boiling point of the oxide, so that higher flame temperatures are also obtained even where the oxide decomposes on vaporization. The combustion of aluminum at 10 atm. results in a flame temperature of 4400" K., compared to 3800" K. at 1 atm. Ignition Temperature of Metals
In general, the rates of oxidation of metals follow the Arrhenius equation; the rate of oxidation is increased with increase in temperature, the exact relationship depending upon the energy of activation of the reaction. As the heat released is directly proportional to the reaction rate, this also exhibits the same temperature dependence. -4 temperature is eventually reached at which the heat generated begins to exceed the heat dissipated to the surroundings, the ensuing autogenous temperature rise brings the material to the glow or flame point, and the metal is said to have ignited. This is termed the ignition temperature and should be a characteristic property of the metal, similar to melting point, density, and tensile strength. The fact that it is not ordinarily so considered is due to the apparent
discrepancies in the values reported by various investigators. However, most of these discrepancies are undoubtedly due to failure to reproduce test conditions in every detail; in all probability once a firm definition of ignition temperature is accepted, the precision and accuracy of successive and independent measurements will be enhanced. I t is already accepted that the factors to be investigated as they affect ignition temperatures are: purity of the metal, gas composition including moisture content, pressure, velocity past the surface, state of subdivision, previous history of the metal, and apparatus and technique. The mechanism of oxidation is also usually affected by temperature, in that the rate law which applies is different for different temperature ranges. I t seems to be generally accepted, a t least for pure metals (although not experimentally verified in every case), that oxidation proceeds exponentially with time at low temperatures, parabolically a t intermediate temperatures, and linearly at high temperatures. The temperature ranges corresponding to these mechanisms are, of course, different for different metals. Cubicciotti (72) has cited for a few metals the temperature at which the oxidation changes from parabolic to linear-for thorium, calcium, and aluminum, 350", 400°, and 500" C., respectively. I t is not clear if there is any correlation between this transformation temperature and the ignition temperature. Thorium and calcium ignite (in oxygen) about 100' C. above this temperature, but aluminum has a n ignition temperature at least GOO0 C. above its parabolic to linear transition temperature. Perhaps this transition must take place before ignition, although such a statement lacks experimental verification. The ignition temperature as defined is dependeht upon the rates of heat generation and of heat dissipation. I t would seem plausible, therefore, to expect a correlation between ignition temperature and the rates of oxidation, at least when the oxidation is linear. This follows from the fact that the higher the rate of linear oxidation, the more heat is produced per unit time and hence the lower the temperature at which ignition can occur (differences in heat dissipation due to differing oxide properties are ignored). Such a correlation has been noted by Fassell and others (74) for various magnesium alloys. The linear oxidation rate of each alloy at 475" C. was tabulated with the corresponding ignition temperature; as the oxidation rate increased, the ignition temperature decreased. Such a correlation, although perhaps applicable to a given alloy system, has not been shown to metals in general.
Table IV. Adiabatic Combustion Temperatures in Oxygen a t 1 -Atm. Pressure
Metal A1 Mg Li Na K Be Ca Sr Ba B Th Ti
Hf Zr Fe
Adiabatic Comb. Temp.,
Metal
OK. 3800 3350 2600 2000 1700 4300 3800 3500 3000 2900 4700 3300 4800 4800 3000
Adiabatic Comb. Temp., OK.
Cr Mo W Mn
3300 3000 3200 3400 2200 1700 2500 2700
Zn Ca Si
Sn Pb Sb Bi
1800
1700 2000
As the ignition temperatures of metals have been studied only sparingly, the factors affecting ignition have not been clearly defined nor has the mechanism been fully explained. However, it is certain that metals can be divided into three classes based on their ignition temperatures: (1) metals that ignite at or below their melting points, (2) the metals that ignite after they melt (magnesium is a n example of Class I and aluminum of Class 11), and (3) metals that have no ignition temperature (silver, mercury, platinum). Some recently measured ignition temperatures are shown in Table V with data of Mellos (27). A single piece of the metal weighing about 10 grams was
Table V.
Ignition Temperatures of Metals in Oxygen Ignition Temp., O C. M. p,, Metal ' C. This work Mellor Li 179 190 180 Na 97.8 118 loo& K 63 69 70a Rb 39 cs 28 ' 3Oa Mg 650 625 540 Ca 850 550 300 Sr 771 720 Ba 720 175 B 2100 700b A1 660 > 1000 580 Ti 1730 ' 610 Zr 1845 1400" Th 1800 500 400b Ta 2996 600 Cr 1900 2000 Mo 2620 750 500-600 Fe 1535 930 Zn 419 900 505b Cd 321 760 Sn 232 865 2700'~~ Pb 327 870 Sb 630 650 ?600asb Bi 271 775 160banb a Estimated from remarks of Mellor. Ignition in air not oxygen.
... ...
...
...
...
...
... ... ... 9 . .
1 . .
...
...
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Table VI. Classification of Metals According to ignition Temperature
Ignition Temp., P. and E. Metal C. C. Ratio Class I, Ignition at or below Melting Point
M,. P.,
Mg
650 1800 850 1535 771 720 2620
Th Ca
Fe
Sr Ba Mo Class
660 419 232 327 271 630 321 179 97.8 63
Zn Sn
Pb
Bi THE METAL Figure 1 1 , Ignition temperatures should be considered a characteristic property, as shown b y the periodicity of the ignition temperatures at 1.0 atm. vs. atomic weight
placed in a n Alundum crucible contained in a Type FD 104 Hoskins electric furnace and heated to a predetermined temperature in a n argon atmosphere (a Chromel-Alumel thermocouple inside the crucible next to the sample gave the temperature). When the temperature became uniform, a stream of oxygen was fed to the top of the crucible to sweep the argon away and expose the metal to oxygen (the velocity of the jet was very low). A sight-glass enabled visual observation of the sample to determine if ignition was obtained. If the sample did not ignite, a new speci-
men was used and the procedure repeated until ignition was obtained (accuracy no better than f l O o '2.). The metals studied are divided into two classes in Table VI and tabulated with their Pilling and Bedworth ratio (28). All Class I1 metals listed have melting points above 650' C , Class I metals have melting points below 660' C. I t appears that the alkali metals belong to Class 11, while the metals of the magnesium group belong to Class I. I t might be reasoned that metals whose Pilling and Bedworth ratio is greater than 1.O (indicating protective oxide film) should not ignite until after they melt and their mobility in the liquid states causes rupture of the oxide film. This reasoning is in accord with Table VI, where all the Class I1 metals indicate a ratio greater than 1.O (improved experimental accuracy might relocate the alkali metals in Class I, although there is no reason why a metal with a nonprotective oxide should not melt before it ignites). The ignition temperatures of the alkali metals decrease from lithium to potassium and a plot of ignition temperature us. atomic number for this group is linear. A similar plot for the magnesium group is also linear, but strontium is much too high, suggesting that this
Table VII.
0.81 1.35 0.64 2.10 0.61 0.67 . I .
TI, Ignition above Melting Point
A1
ATOMIC NUMBER OF
623 500 550 930 720 175 750
Sb Cd
Li Na
K
1000 900 865 870 775 650 760 190 118 69
1.28
1.55 1.32 1.31 1.20 1.44 1.21 0.58 0.55 0.45
value may be in error. The ignition temperature of strontium from the plot should be about 360' C. instead of 720' C. as given in Table V. That ignition temperatures should be considered a characteristic property is supported by the periodicity shown in Figure 11. This variation is similar to that noted in similar plots of melting and boiling points and heats of formation. Powdered-Metal Flames
Powdered metals dispersed in oxygen or air form explosive mixtures with ignition temperatures lower than those of the corresponding bulk metal. As might be expected, these mixtures have ignition temperamres; limits of flammability, and minimum energies for ignition just as do the explosive mixtures formed by the gaseous combustibles such as acetylene, propane, and hydrogen. Their flammability and explosibility have been reported in considerable detail (19-27), in an attempt to evaluate all factors affecting flame propagation. The similarity between powdered metal-oxygen (or air) dispersions and combustible gas-oxygen (or air) mixtures
Operating Data for Powdered-Metal Flames Min.
Explosive
c ~ , G~~~ ~ ~ Stoichiometric ~ , , Composition Meta A1
Mg
Oxide Formed
Ignition Temp., C.
Metal/Liter Air
G. metal/ 1. 0 2
G. metal/ g. 02
A1203 MgO
645 520 315
0.025 0.020 0.105 0.105 0.045 0.045 0.16 0.21 0.04
1.6 2.14 5.0 3.33 2.14 2.87 1.25 4.9 4.1
1.12 1.5 3.5 2.3 1.5 2.0 0.88 3.43 2.87
Fe
FeO Fen03
Ti
Ti02 Ti201
Figure 12. Early model of powdered metal feed device
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Si Mn Zr
Si02 MnO ZrO?
... ...
480
775 450 Room temp.
METAL COMBUSTION IN OXYGEN suggested the operation of flames similar to the Bunsen and oxyacetylene flames. Early studies indicated that basically the successful operation of a flame consuming a powdered metal-oxygen mixture depended upon four factors: 1. A device capable of feeding metal powders at a constant and reproducible rate 2. Dispersion of the powdered metal into the air or oxygen to form a flammable mixture 3. Ability of the air or oxygen required for combustion to convey the powder successfully through the transport lines to the torch 4. Ignition of the dispersion as it leaves the tip of the torch A simple and easily constructed feed system is shown in Figure 12. The powdered metal storage tank, A, is connected by a T-fitting to a IO-inch length of I-inch pipe. A small screw conveyor housed inside the pipe connects through a stuffing box to the drive motor. Another T-fitting is attached tq the delivery end of the 10-inch pipe and is fitted with a short length of pipe, B, which leads to the mixing chamber, C. The interior of C is streamlined, so that the powder delivered down the shaft, B, is swept clear by the metered oxygen stream which enters at D. The dispersion leaves through a grounded transport line, E, leading to the torch. The system is equalized from F to G and a small vibrator attached to the side of the feed tank keeps the powder flowing freely, so that the feed rate can be varied simply by changing the speed of the screw conveyor. The design of the mixing chamber does not allow for powder accumulation; hence all the powder leaving the screw conveyor is mixed with a given quantity of oxygen to form the desired dispersion. The stoichiometric concentrations employed in these studies are well above the lower limits of flammability, as shown in Table VI1 (upper explosive limits not reported). The stoichiometric quantity of oxygen required for combustion is adequate to convey the powders through the transport line leading to the torch. The most common form of pneumatic conveying systems requires air velocities of the order of 50 to 100 feet per second to convey material in the ratio of 5 grams of solid per gram of air. The metaloxygen ratio (stoichiometric) for most metals is slightly less than this; it was satisfactory in these studies to maintain a velocity of about 50 feet per second in the transport line. The highly explosive dispersion leaving the tip of the torch will ignite if a lighted match is placed close to the face of the torch. A stable or stationary flame is not obtained (as with Bunsen and oxyacetylene-type flames) ; once the match
Figure 13.
Early model of oxyaluminum torch in operation
is removed the flame will no longer persiit. A continuous pilot flame must be provided, if this type of torch is to be operated continuously. I n a typical torch design small pilot-gas holes completely encircle the central, dispersioncarrying conduit. In operation the pilot gas (propane, acetylene, hydrogen, carbon monoxide) is fed to the rear of the torch, flows to a manifold and then to the pilot-gas holes at the face of the torch to form a sheath of flame around the central tube (pilot gas rate with propane was about 14 liters per minute). This design provides for rapid ignition of the mixture leaving the torch. Torches with central tube diameters varying from 0.125 to 1 inch have been operated with metal rates from 50 to 550 grams per minute. It is also possible to operate with the powder carried by the pilot gas (oxygen coming out of the pilot-gas holes) and eliminating the pilot-gas holes, with powder carried by the pilot gas-oxygen mixture. I t has been necessary to use powders finer than 200 mesh to obtain rapid and efficient combustion. Experiments have been made using aluminum, magnesium, iron, cast iron, manganese, silicon, titanium, zirconium, calcium, magnesium-aluminum alloy, calcium-silicon alloy, zirconium-silicon alloy, aluminumsilicon alloy, and various carbides. Typically, powdered metal-oxygen flames are highly luminous (Figure 13), and if viewed with dark glasses (No. 4 or 5) are seen to have the characteristic
WAT'ER-COOLED
structure shown in Figure 14. The flames ignite a short distance, C, away from the face of the torch, producing a n intensely brilliant flame containing a very bright and diffuse zone. C varies with the metal used and the linear velocity (at low velocity C is essentially zero, as the flame ignites close to the face of the torch, but in some cases C was about 4 inches). The length of the bright zone, the hottest portion of the flame, varies from 1 to 12 inches and its diameter from 0.5 to 3 inches, depending on the powder feed rate. The total length of the flame depends on operating conditions and may be as long as 48 inches. These flames generate large quantities of smoke consisting of metallic oxide particles (Figure 13). The linear velocity through the torch must be maintained above a certain minimum to prevent flash back. Flashback velocity depends on the metal and its particle size and is about 20 feet per second for Alcoa 101 aluminum powder and 30 feet per second for magnesium powder of the same particle size. Reynolds 400 aluminum powder (average particle size, 4 to 6 microns) has a flash-back velocity of about 100 feet per second. The blowoff velocity too depends upon the metal and its particle size, but is also affected by the pilotgas rate. Using Alcoa 101 the torch , can be operated satisfactorily (pilot gas rate about 14 liters per minute) up to 300 feet per second. Above this velocity the flame no longer has the structure
TORCH Figure 14.
Powdered metal-oxygen flame VOL. 50, NO. 4
APRIL 1958
671
shown in Figure 14, but appears as a shower of sparks yielding poor combustion efficiencies. A torch design in which the pilot-gas orifices were directed toward the axis of the torch gives good flames with Reynolds 400 aluminum powder a t velocities up to 975 feet per second. Zirconium is very flammable and special techniques have been developed to burn these dispersions. Powdered metal-oxygen flames are effective in melting various ceramic materials. This effectiveness is attributable to the following factors: Very high temperatures are obtained (over 3000 O C in the case of aluminum) Large quantities of heat are generated in a short time. Combustion takes place in close proximity to, if not on, the surface being me1ted . Flames are highly luminous and hence emit large quantities of radiant energy. Combusrion produces metallic oxides, usually in the vapor or liquid state, and condensation of the vapors and the slagging action of the liquid give rise to rapid heat transfer. Physical nature of the flames enables a certain amount of erosion to occur. Y
,
These torches have been used to cut a 3-inch hole through 30 inches of concrete with a penetration rate of more than 1 inch a minute. The flame will penetrate an aluminum oxide brick ( 2 inches thick) in less than a minute (watercooled torches are necessary to prevent the intense, heat from causing ignition within the central tube of the torch; the linear velocity must be kept above 100 feet per second to guarantee safe operation). The magnesium-oxygen flame shows a n unusual type of behavior. The combustion leads to the formation of magnesium oxide, melting point 2800’ C. This is very close to the temperature of the flame and hence very little molten oxide is produced. The powdery oxide builds up on the surface of the brick, insulating it from the flame, and the brick melts very little If silicon powder is admixed with the magnesium powder, the low melting magnesium silicate is formed and the brick is easily melted. The aluminum-oxygen flame (using Alcoa 101 aluminum powder) has been operated a t pressures u p to 60 p.s.i.g. I n these tests (the torch and feed device were the same as in the atmosphere pressure runs) the torch was slipped through a stuffing box arrangement to discharge into a pressure chamber. I n starting a run the torch was ignited at atmospheric pressure and the valve on the exit line of the pressure chamber was wide open. The valve was then closed slowly to build u p to the desired operating pressure. This simple technique depended on installing a properly sized orifice in the oxygen line just upstream of the mixing chamber (C, Figure 12) and
672
running it choked with a back-pressure of 200 p.s.i.g. The downstream pressure (in the test chamber) could then be increased to about 100 p.s.i. (critical pressure ratio is actually about 0.53) without affecting the mass flow rate, although the maximum test pressure was 60 p.s.i.g. (pilot-gas was carbon monoxide to prevent accumulation of condensed water vapor within the test chamber and it was similarly throttled). This simple technique allowed easy operation of this flame at elevated pressures. T h e purge line, FG (Figure 12), made the feed device insensitive to the operating pressure level. The mass flow rate of the oxygen (and pilot gas) remains constant as the chamber pressure is increased, as long as the critical pressure is not exceeded. However, the linear velocity through the transport line and through the torch itself decreases as the pressure .is increased, because of the density increase. This was anticipated and provisions were made in the design of the test procedure. For instance, a t atmospheric pressure the torch was operating a t about 250 feet per second, which a t 60 p.s.i.g. reduced to about 50 feet per second a t the same mass flow rate due to the density increase. This is above the flash-back velocity for this powder dispersion. Although not measured precisely, indications were that the flash-back velocity a t 60 p.s.i.g. was not appreciably different from that at atmospheric pressure. Acknowledgment
After initiation of high temperature research a t the institute, the program was financially supported by the Office of Naval Research for a number of years. Since then it has received financial support from the Office of Ordnance Research, and lately from the Air Force Office of Scientific Research, Air Research and Development Command. The support of all of these government agencies is gratefully acknowledged. The authors also wish to acknowledge the helpful assistance of R . A. Miller, A. D. Kirshenbaum, M. S. Kirshenbaum, Theron Lee, Jr., Joseph Nelli, W. J. Liddell, and IVilliam Stoerrle in the experimental portions of this study. literature Cited (1) Baker, R. A., Strong, F. M., IND. ENG.CHEM.22,788 (1930). (2) Benedict, W. S., Bullock, B. W., Silverman, S., Grosse, A. V., J . O j t . Soc. Am. 43, 1108 (1953). (3) Brewer, L., Chem. Revs. 52, No. 1,
1-75 (1953). (4) Bunsen, R., Roscoe, H. E., Phil. Trans. 149,920 (1859). (5) Cassel, H. M., Das Gupta, A. J., Guruswamy, S., Third Symposium on Combustion, Flame and Explosion Phenomena, p. 185, Williams 8r. Wilkins, Baltimore, 1949.
INDUSTRIAL AND ENGINEERING CHEMISTRY
(6) Coffin, K. P., Natl. Advisory Comm. Aeronaut. TN 3332 (1954). (7) Conway, J. B., Grosse, A. V., 2nd Technical Report, High Temperature Project, Contract N9-onr87301, “Aluminum Sun Furnace,” Research Institute of Temple University, July 1, 1952. (8) Ibzd., 3rd Technical Report, “Powdered-Metal Flames,” Aug. 1, 1953. (9) Conway, J. B., Miller, R. A , , Grosse, ,4.V., Research Reas. ONR (August 2951), pp, 1-6. (10) Conway, J. B., Smith, W. F. R., Liddell, W. J., Grosse? A. V., J . Am. Chem. SOC.77,2026 (1955). (11) Conway, J . B.? Wilson, R. H.: Jr., Grosse, A. V., Ibid., 75, 499 (1953). (12j Cubicciotti, D. D.> Jr., Iron Age 171.No. 21. 144 (1953’1. (13) Cueilceron, J.; Scar&&, H., Compt. rend. 228, 489 (1949). (14) Fassell, W. M.. Jr., Gulbransen, L. B., Lewis, J. R., Hamilton. J . H., J . ,Welais 3, N o . 7 , 521 (July 1951). (15) Forsvthe. W. E., Easlev, hi. A . , ~.‘o$t: sac. Am. 21, 685 ii93i). (16) Glassner, .4., Argonne National Laboratory, ANL-5107 (August 1953), superceded by ANL-5750. (17) Grosse, A. V., Conway, J. B., 1st . Technical Report, High Temperature Project, Contract Ng-onr87301, “Combustion cf Metals,” Research Institute of Temple University, Oct. 15, 1951. (18) Hart, D., Tomlinson, W. K., Jr., M e t a l P r o p . 59, 788 (1951). (19) Hartmann, I., IND.EXG.CHEM.40, 752 (1948). (20) Hartmann, I., Nagy, J., Brown, H. K., L. S. Bur. Mines, R.ept. Invtst. 3722 (October 1943). (21) Hartmann, I.: Nagy, J., Jacobson, M., Ibid., 4835 (December 1951). (22) Kelley, K. K., U. S. Bur. Mines, Bull. 476 (1949). (23) Kubasehewski, O., Catterall, d . A . , “Thermochemical Data of Alloys,” Pergamon Press, London, New York, 1956. (24) Kubaschewski, O., Hopkins, B. E., “Oxidation of Metals and Alloys,” Academic Press, New York, 1953. (25) Liempt, J. A. M. Van, Vriend, J. A. de, Rec. trao. chim. 53, 839, 895, (1934); 56, 126 (1937); 58, 423 11939L (26) Lippman, E. 0. von, Chem. Z’tg. 32, 161 (1908). (27) Sfellor. J . W.. “Treatise on Inorganic and Theoretical Chemistry,” Longmans, Grcen & Co., Sew York. 1946. (28) Pilling, N. B., Bedworth, R. I
